Plasma Chemistry II

112 Pages·1980·1.866 MB·English

by  Mundiyath Venugopalan, Uptal K. Roychowdhury, Katherine Chan (auth.)

0 scipoT ni tnerruC yrtsimehC
Fortschritte der Chemischen Forschung
Plasma Chemistry II
Editors: .S Vep~ek and .M Venugopalan
Springer-Verlag
Berlin Heidelberg New York 1980
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Contents
Plasma Chemistry of Fossil Fuels
M. Venugopalan, U. K. Roychowdhury, K. Chan, and M. L. Pool
Kinetics of Dissociation Processes in Plasmas
in the Low and Intermediate Pressure Range
M. Capitelli and E. Molinari 59
Subject Index 111
Author Index Volumes 26-90 113
Editors of this volume:
Dr. Stanislav Vepfek, Anorganisch-Chemisches Institut der Universitiit, Winterthurerstrafie 190,
CH-8057 Ziirich
Prof. Dr. Mundiyath Venugopalan, Department of Chemistry, Western Illinois University,
Macomb, IL 61455, USA
Editorial Board:
Prof. Dr. Michael .3 S. Dewar, Department of Chemistry, The University of Texas, Austin,
"IX 78712, USA
Prof. Dr. Klaus Hafner, Insitut f/Jr Organische Chemie der TH, Petersenstrage 15,
D-6100 Darmstadt, FRG
Prof. Dr. Edgar Heilbronner, Physikalisch-Chemisches Institut der Universit~it, Klingelberg-
straBe 80, CH-4000 Basel
Prof. Dr. OhS Itd, Department of Chemistry, Tohoku University, Sendai, Japan 980
Prof. Dr. Jean-Marie Lehn, Institut de Chimie, Universit6 de Strasbourg, .1 rue Blaise Pascal,
B. P. 296/R8. F-67008 Strasbourg-Cedex
Prof. Dr. Kurt Niedenzu, University of Kentucky, College of Arts and Sciences, Department of
Chemistry, Lexington, KY 40506, USA
Prof. Dr. Charles .W Rees, Hofmann Professor of Organic Chemistry, Department of Chemistry,
Imperial College of Science and Technology, South Kensington, London SW 72 AY, England
Prof. Dr. Klaus Schiller, Institut f/ir Physikalische Chemie der Universit~it, Im Neuenheimer
Feld 253, D-6900 Heidelberg 1, FRG
Prof. Dr. Georg Wittig, Insfitut fiir Organische Chemie der Universit/it, Im Neuenheimer
Feld 270, D-6900 Heidelberg 1, FRG
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Plasma Chemistry of Fossil Fuels
Mundiyath Venugopalan, Uptal K. Roychowdhury,
Katherine Chan, and Marion L. Pool
Department of Chemistry, Western Illinois University, Macomb, Illinois 61455, U.S.A.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . 2
2 Thermodynamic and Kinetic Aspects of Fossil Fuel Chemistry . . . . . 3
3 Natural Gas and Methane Plasmas . . . . . . . . . . . . . . 4
3.1 Low Frequency Discharges . . . . . . . . . . . . . . . 4
3.2 Triboelectric Discharges . . . . . . . . . . . . . . . . 6
3.3 High Frequency Discharges . . . . . . . . . . . . . . . 6
3.4 Electrical Arcs and Plasma Jets . . . . . . . . . . . . . . 10
3.5 Laser Irradiation . . . . . . . . . . . . . . . . . . 14
4 Petroleum and Petroleum By-product Plasmas . . . . . . . . . . 14
4.1 Low Frequency Discharges . . . . . . . . . . . . . . . 51
4.2 High Frequency Discharges . . . . . . . . . . . . . . . 61
4.3 Electrical Arcs and Plasma Jets . . . . . . . . . . . . . . 17
4.4 Submerged Arcs in Liquid Petroleum . . . . . . . . . . . 19
4.5 Laser Irradiation . . . . . . . . . . . . . . . . . . 22
4.6 Plasma Desulfurization of Petroleum . . . . . . . . . . . . 22
5 Plasmas in Coal . . . . . . . . . . . . . . . . . . . . 24
5.1 Low Frequency Discharges . . . . . . . . . . . . . . . 25
5.2 High Frequency Discharges . . . . . . . . . . . . . . . 26
5.3 Electrical Arcs and Plasma Jets . . . . . . . . . . . . . . 32
5.4 Flash and Laser Irradiation of Coal . . . . . . . . . . . . 38
5.5 Plasma Gasification of Coal . . . . . . . . . . . . . . . 43
5.6 Plasma Desulfurization of Coal . . . . . . . . . . . . . . 44
6 Other Fossil Fuel Plasmas . . . . . . . . . . . . . . . . . 46
7 Concluding Remarks . . . . . . . . . . . . . . . . . . 49
8 References . . . . . . . . . . . . . . . . . . . . . . 50
.M nalapogmreV et La
1 Introduction
Fossil fuels are the subterranean remains of green plants and animals that once grew
and then were buried in sedimentary sands, muds and limes under conditions of in-
complete oxidation. The present supply of fossil fuels includes coal, oil, natural gas,
oil shale and tar sands. Natural gas si the fossil fuel in shortest supply and greatest
demand. The simple hydrocarbon methane is the predominant component and rep-
resents 80-95 volume percent of any natural gas. Over the years several techniques
have been applied to produce methane from other fossil fuels such as petroleum
which is a mixture of hydrocarbons with six or more carbon atoms and coal which
is a complex mixture of some organic compounds. One such technique si the produc-
tion of a plasma in petroleum and coal through the action of either very high tem-
peratures or strong electric fields. Since coal is in greatest supply the objective in-
cluded obtaining new knowledge of coal chemistry, which may lead to new methods
of producing organic chemicals.
The properties of the plasma produced in fossil fuels vary widely. Those plasmas
labeled "glow discharges" are characterized by average electron energies of 1-10 eV,
electron densities of 1015-1018 m -3 and lack thermal equilibrium in the sense that
electron temperatures are much greater than gas kinetic temperature
(Te/Tg = 10-100). The absence of thermal equilibrium makes it possible to obtain
a plasma in which the gas temperature may have near ambient values while at the
same time the electrons are sufficiently energetic to cause the rupture of molecular
bonds. It is this characteristic which makes glow discharges well snited for the study
of chemical reactions involving thermally sensitive materials such as petroleum and
natural gas. By contrast, plasmas labeled "arcs" or "jets" have nearly identical elec-
tron and gas temperatures (> 5000 K). The high gas temperature makes these plas-
mas suitable for producing chemicals by degrading complex organic materials such
as coal, shale and tar. The highly excited species that exist in these plasmas can react
to produce compounds whose formation is thermodynamically unfavorable under
ordinary experimental conditions. The physical and chemical properties and the pro-
duction of both types of plasmas have been fully described elsewhere .)1
Because of the complex structure of coal and the variable composition of petro-
leum most of the plasma work using these materials is descriptive in nature. Attempts
at modeling have been confined to the carbon-hydrogen system, chiefly using graphite,
perhaps due to its importance in nuclear fusion and as aerospace material. In this
chapter the studies of coal, petroleum hydrocarbons and natural gas in glow dis-
charges, electrical arcs and jets are reviewed. Also reviewed are those studies in which
these fossil fuel plasmas are formed in presence of simple gases such as hydrogen,
nitrogen and argon. A comparison is then made with those studies in which lasers
and flash heating techniques were applied. Pertinent investigations on the structural
aspects of plasma-treated coal are included. Finally, the status of work on plasma
desulfurization and gasification of coal and petroleum is discussed.
amsalP Chemistry of Fossil Fuels
2 Thermodynamic and Kinetic Aspects of Fossil Fuel Chemistry
Thermodynamic considerations of the carbon-hydrogen system provide a useful guide
to the nature and yield of products which might be obtained from fossil fuels at the
temperatures attained in various plasma devices.
At temperatures between 900 and 2000 K most hydrocarbons have a positive
free energy of formation, which, with the exception of acetylene, increases with in-
creasing temperature .)2 Below 500 K only the paraffinic hydrocarbons are thermo-
dynamically stable. Above 1700 K acetylene has a lower free energy of formation
than the other hydrocarbons, but it is still thermodynamically unstable. Consequently,
acetylene can be obtained by rapidly carbonizing fossil fuels at about 1800 K, but
the yield is still mainly governed by chemical kinetics .)3 That is to say, the reaction
time must be sufficiently long to permit the decomposition of other hydrocarbons
to acetylene, but sufficiently short to prevent any appreciable decomposition of the
acetylene formed to carbon and hydrogen. At temperatures of about 4000 K, the
free energy of formation of acetylene from its elements approaches zero, and the
equilibrium yield of acetylene is appreciable. The system is complicated, however,
by other reactions and phase changes which occur at these high temperatures. For
example, carbon sublimes at about 4000 K, molecular hydrogen dissociates, and
various species such as C, C 2 and C 3 are formed.
Coals, particularly the bituminous and sub-bituminous varieties, undergo primary
decomposition in the temperature range of 700-800 K. If coal carbonization could
attain thermodynamic equilibrium over this temperature range, the hydrocarbon
products with the exception of methane, if any, would be decomposed mainly to
carbon and hydrogen. In practice, thermodynamic equilibrium is not attained, and
the composition of the hydrocarbon by-products si mainly determined by the tem-
perature and the kinetics of the process.
The equilibrium between carbon and hydrogen at high temperatures has been
treated thermodynamically by several authors 4' s). The approach was to formulate
the various reactions which could possibly occur, to apply to each the appropriate
mass action equation, and to solve the set of simultaneous equations so obtained.
A distinction was made between heterogeneous and homogeneous systems, since for
the latter it is necessary to specify the mole ratio of carbon to hydrogen (C/H) in the
system.
Assuming that the equilibrium composition of a reaction mixture with C/H = 1.0
at 2000-5000 K would consist of C, C2, C3, ,sC H, H2, CH, C2H, C3H, C4H, CH2,
C2H2 and C4H2, Baddour and Blanchet s) found that the mole fraction of acetylene in the
equilibrium mixture passes through a maximum with temperature, the value being
0.07 at 3300 K while that of C2H si 0.1 at 3800 K. To apply this information to the
products obtained at room temperature they assumed that C 2 H2 remained unchanged
on quenching, while C2H recombined with H to yield more C2H 2. On this basis, the
theoretical maximum acetylene concentration in the quenched gas was found to de-
pend on the temperature and C/H ratio of the system: At 3200 K, with a C/H ratio
of 0.25 (as for CH4) the maximum volume percentage of acetylene in the quenched
gas is 12; with a C/H ratio of 0.50, the amount of acetylene can be increased to 19%.
.M nalapoguneV et .la
At 4300 K and a C/H ratio of 15, the maximum concentration is 50%, a value con-
siderably higher than those found in experiments using high intensity arc reactors.
The kinetic aspect of the reaction between graphite and hydrogen has been
studied 6). At temperatures above 3000 K, the sublimation of graphite is the control-
ling factor, and the reaction rate is independent of hydrogen pressure provided that
there are sufficient hydrogen molecules to react with all the Cn species that evaporate.
Below 3000 K, the reaction of graphite and hydrogen between 0.01 and 1 atm is a
surface reaction whose rate is proportional to the hydrogen pressure and the square
root of the dissociation constant of hydrogen. Several authors have investigated the
reaction of graphite in low pressure discharges at relatively low temperature of hy-
drogen. Veprek and coworkers )7 have shown that both the diffusion of H atoms
towards the carbon surface and the diffusion of reaction products from the surface
are much faster than the rate of surface recombination and the surface reaction. For
pyrolytical graphite the probability of the reaction defined as the ratio of the num-
ber of C atoms leaving the surface to the number of H atoms impinging on the sur-
face is about 10 -4 or less. Since the reaction probability would depend strongly on
the quality of the carbon used, it would be much higher for carbon of a poor quality,
such as that found in coal.
Under plasma conditions any oxygen present in the coal will be evolved as car-
bon monoxide. If the carbonization is carried out in nitrogen atmosphere, acetylene
and hydrogen cyanide will be the main products. Very little cyanogen would be
formed unless the nitrogen is greatly in excess of the hydrogen in coal .)8 For a dis-
cussion of the thermodynamics of the C-H-N system the reader is referred to an
article by Timmins and Ammann .)9 The chemical evaporation, transportation and
deposition of carbon in low pressure discharges of oxygen, nitrogen and hydrogen
have been described recently 1~
3 Natural Gas and Methane Plasmas
Natural gas as obtained from underground deposits generally has a composition that
si significantly different from that of the familiar commercial fuel. The crude gas
usually contains some undesirable impurities such as water vapor, hydrogen sulfides,
and thiols or other organic sulfur compounds in addition to some heavy, condens-
able hydrocarbons 11). Appropriate processing eliminates or reduces the amount of
the undesirable impurities and allows the condensable hydrocarbons to be collected
as a separate fraction of industrial value. The following volume composition for the
commercial fuel is thus arrived~2): 80-95% CH4, 8-2% C2H6, 3-1% C3H8,
< 1% C4H10 , < %1 CsH12 , 10-0% N2, <2% CO2. The concentrations of the minor
components vary slightly with the source of the gas.
3.1 Low Frequency Discharges
The decomposition of methane in the glow discharge has been investigated for many
years. At low pressures ethane was the major product la). As the methane pressure
4
amsalP Chemistry of Fossil Fuels
was increased, ethylene and acetylene were formed; their concentrations in the prod-
uct became significant if the reactor was cooled in liquid air 14-16). In a flow
system at atmospheric pressure under conditions of high conversion and relatively
high temperatures Wiener and Burton )71 found that the yield of acetylene was quite
high.
In the negative glow )81 of a dc discharge in methane at low pressures
(0.05-0.3 torr) and low currents (0.1-5 mA) ethane, ethylene and acetylene were
found in addition to hydrogen and the nonvolatile cuprene, (CH)n, which appeared
mainly on the cathode as a solid. Lowering the temperature of the reactor from
77 K to 63 K greatly increased the amount of ethylene. Smaller amounts of propane,
propene, propyne, butane, butene, butadiene and pentene were also found. Their
rates of formation increased with increasing discharge current at the expense of the
2C hydrocarbon products and cuprene. The addition of hydrogen to the methane
had little effect on the products. Variation of the inter-electrode separation indicated
that the products were not formed uniformly throughout the negative glow.
Recently a movable glow discharge of methane has been investigated mass spec-
trometrically over the whole column length 19). The mass spectra showed primary
fragment ion of methane and ions from condensation reactions up to m/e = 113.
The current of different ions reached a maximum very close to the cathode and
varied regularly along the axis in maximum and minimum which were related directly
with the striations of the column. By simulating the conditions of the glow discharge
in a mass spectrometer with high pressure ion source the same ion-molecule reactions
were identified with which it was possible to explain the formation of condensation-
type ions in the discharge.
Methane has been decomposed in ac discharges operated using 1-6 kV and
30-70 mA in flow systems at pressures of 1-10 torr 2~ With contact times of
0.05-1.5 s the principal products were acetylene, ethane, ethylene and hydrogen
together with some higher molecular weight compounds. The conversions which
varied from 28 to 91% increased on increasing the contact time and/or discharge
current. However, the yields of 2C and 3C hydrocarbons reached a maximum at
40-50% total conversion. Almost complete conversions of methane to acetylene
have been reported in later works 21' 22). Vishnevetskii et alfl 3-2s) have given a set
of equations for calculating the rates of acetylene and ethylene formation and the
rates of decomposition of several hydrocarbons.
Several attempts 26-31) have been made to analyze the numeroushigher molec-
ular weight products from line frequency spark and pulsed discharges in methane
at pressures of 250-500 torr. The work is of great interest in connection with the
chemistry of primitive earth atmosphere and the origin of life.
The kinetics of conversion of methane to acetylene in glow discharges has been
studied in detail 32-3s). A great amount of this work has centered around such para-
meters as power yields, pressure, cell design, electrode material and presence of
hydrogen or argon. Methane conversion was hindered by H2 or Ar to the same ex-
tent and was greater with greater partial pressure of these added gases in the pres-
sure range 40-150 torr. However, the cracking of methane was accelerated by H 2
and Ar at a total pressure of 10 torr. Compared with the higher paraffins, methane
yielded the least amount of acetylene. The cracking rate constant increased with
.M nalapoguneV et .la
pressure. With increasing specific power (power per unit volume of input methane)
consumption the degree of conversion increased gradually on inactive and little car-
bonized, but rapidly on active and carbonized electrodes.
Glow discharges at low pressures in mixtures of methane and nitrogen produced
acetylene simultaneously with hydrogen cyanide 36). 2C H2/HCN ratios varied accord-
ing to the current densities and mixture compositions used. When mixtures of me-
thane and carbon dioxide or methane and water were subjected to discharge, carbon
monoxide and hydrogen were produced. Acetylene was found only at low current
densities .)73
The production of acetylene from natural gas has been studied in a 60 Hz elec-
trical discharge at atmospheric pressure 38). The apparatus consisted of a cyclone-
type reactor in which the products could be removed through a hollow electrode.
The electrode separation was 2 cm; the potential difference necessary for spark over
gas was 13-16 kV. By varying the residence time of the gas in the reactor from 30
to 600 s a product containing 15-17% 2C H2 by volume was obtained at relatively
low temperatures. Increasing the input rate and decreasing the specific power con-
sumption increased the amount of C2H 2 formed, but decreased the C2H2 concen-
tration in the product. Copper electrodes were reported to produce the least depo-
sition of carbon black and cuprene.
3.2 Triboelectric Discharges
Methane has been converted into ethane, ethylene, acetylene and hydrogen in a
"triboelectric" discharge arising from the intermittent contacting of mercury with
a glass surface '93 40). The discharge is a result of the accumulation of high densities
of static charge at the interface by the transfer of electrons from the mercury to the
glass. Spectroscopic studies of the discharge have indicated that excited species with
energies up to 20 eV above their ground states are present. Further, the tribolumines-
cence spectrum differed from the spark discharge murtceps at atmospheric pressure
in that 2C emission was absent. Both area and nature of the solid surface influenced
the extent of breakdown and discharge. The rates of methane conversion were vir-
tually invariant with pressure from 760 to 200 torr, but at 200 torr they increased
sharply before gradually falling off again as the pressure was further reduced. The
addition of 10% noble gases did not result in any pronounced change in the 2C hy-
drocarbon yields or affect the product distribution which was
C2H6: C2H 4 :C2H 2 = 1:0.34:0.32.
3.3 High Frequency Discharges
Eremin 40 reported that the amount of methane cracked by a high frequency dis-
charge is proportional to the current consumed and to the amount of excess hydro-
carbons. The reaction was found to be of the first order and the rate was directly
proportional to the discharge energy and inversely proportional to the original amount
6